1. Field of the Invention
The present invention relates to the field of switch-mode voltage regulators.
2. Prior Art
Switch-mode voltage regulators have inherent limitations in their ability to maintain a constant output voltage in the face of fast load transients. This is particularly true when they are used to power the new generation of microprocessor ICs, which can change their load current demand from hundreds of milliamps to tens of amps in less than 100 nanosec. Switch-mode regulators have difficulty in supplying such load transients due to both the filtering effect of inductors and transformers used in them, and also the parasitic effective series resistance of the output filter capacitors which cause a nearly instantaneous I.times.R output voltage drop when hit with a sudden load current surge.
The standard methods for dealing with these problems are to place additional parallel capacitors at the output to reduce the parasitic effective series resistance of the output filter capacitors, and to decrease the inductance values in the switch-mode regulator. However these solutions are unfavorable in cost and size for the large and sudden transients encountered with the new generation microprocessor ICs. Another method that has at least been suggested is to couple a linear regulator in series from the switch-mode regulator output voltage to the load. This approach should operate well in the presence of occasional large load transients, but the linear regulators are very inefficient and get quite hot if the load transients happen with great frequency, as is the case with modern microprocessors that employ halt-clock and stop-grant states. A linear regulator may be coupled in parallel with the switch-mode regulator, but here again the linear regulator gets hotter than the switch mode regulator, and is inefficient.